1. Technical Field of the Invention
The present invention relates to magnetic resonance imaging based on a magnetic resonance phenomenon in an object to be examined to obtain images of the object, and in particular, to the magnetic resonance imaging designed to perform a pulse sequence based on an encoding technique known as “Elliptic Centric (or Swirl)” preferable to contrast enhanced MRA (Magnetic Resonance Angiography) adopting a dynamic scan performed with an MR (Magnetic Resonance) contrast agent injected into the object.
2. Description of Prior Art
Magnetic resonance imaging is based on a technique that magnetically excites nuclear spins in an object placed in a static magnetic field with an RF signal of a Larmor frequency of the nuclear spins, acquires an MR signal emanated from the object due to the excitation, and reconstructs an image using the MR signal.
In order to perform this magnetic resonance imaging, it is required to use a train of pulses, that is, a pulse sequence that has a variety of types of pulses lined up in time in accordance with a predetermined rule. Based on this pulse sequence, an RF pulse is transmitted from an RF coil to an object to be examined, and an echo signal (MR signal) emanated in response to this transmission from the object based on a magnetic resonance phenomenon of nuclear spins in the object. The received echo signal is then subjected to post-processing to convert it into echo data. If an image reconstruction algorithm relies on a Fourier transform, each echo data is mapped in a frequency space (k-space) at its encoded position defined by a specified amount of an encoding gradient pulse. A set of data filled in this frequency space is then applied to the Fourier transform, so that the data is reconstructed into a real-space image.
One type of magnetic resonance imaging is MRA (magnetic resonance angiography). This MRA can also be categorized into various types, which have been researched actively at present. One type of MRA is known as a contrast enhanced MRA technique that utilizes an MR contrast agent injected into an object to be examined. The contrast agent causes the enhancement in contrast of an image. For performing the contrast enhanced MRA, a dynamic scan is normally carried out, which allows a desired object's region to be scanned repetitively at intervals.
In the contrast enhanced MRA, it is preferred to conduct an encoding technique that gives priority to a central region of a view presented by an image, because a desired blood vessel to be observed, which shows a sharp increase in the contrast due to incoming of an injected contrast agent into the scanned region, is made to locate at the central region. As one type of this encoding technique, there is known “an Elliptic Central (or Swirl)” technique, which uses a pulse sequence in which encoding amounts are set to acquire data to be mapped in a central part of the k-space (i.e., frequency space) immediately after starting a scan.
Meanwhile, in the case of the contrast enhanced MRA, securing high temporal resolution is another factor which should be managed with high attention. From this viewpoint, a further sampling technique has been developed, wherein each unit of acquisition (i.e., RF excitation) in a pulse sequence is determined so that data to be mapped in a central part of the k-space is acquired more than data to be mapped in the remaining part thereof. This sampling technique is sometimes referred to as a “3Dtrics (or DRKS (Differential Rate k-space Sampling)” technique. Applying the processing on a view share technique to the data acquired using this sampling technique gives high temporal resolution to the dynamic scan.
This 3Dtrics technique performed in combination with the foregoing Elliptic Centric technique is exemplified by a reference “ISMRM 2000 No. 1799.”
It is frequent that the foregoing MRA is carried out using a pulse sequence for a three-dimensional FFE (Fast FE) method. This pulse sequence often includes a desired-purpose pre-pulse, such as fat suppression pulse, applied before applying a radio frequency (RF) excitation pulse (flip pulse). However, applying the pre-pulse to an object before all of the RF excitation pulses results in a longer scan time. One countermeasure to remove such an inconvenience is provided by Japanese Patent Laid-open (KOKAI) No. 2001-170023. As proposed therein, the frequency of application of a pre-pulse is changed as the encoding position is shifted outward from the zero-encoding position in the k-space. This adjustment of application frequency of the pre-pulse has been reduced into practice in combination with the foregoing Elliptic Centric technique.
The application technique of the pre-pulse disclosed by Japanese Patent Laid-open (KOKAI) No. 2001-170023 would be, if performed, a help for shortening the scan time and obtaining the effect from a desired pre-pulse. However, in applying such application technique to the contrast enhanced MRA based on a three-dimensional (3D) FFE method which has become popular recently, there is a problem that the contrast enhanced MRA cannot be finished within a period of time during which an object (patient) is usually able to temporarily hold his or her breath without interruptions (i.e., object's one-time continuous breath hold period). The breath-hold technique is significant in removing object's motion artifacts from reconstructed MR images.